PROBLEM TO BE SOLVED: To provide a water treatment apparatus in which growth of a MAP (magnesium ammonium phosphate) crystal is suitably promoted while using a phosphate ion in the water to be treated to efficiently recover phosphorus from the water to be treated. SOLUTION: The water treatment apparatus includes: a phosphorus recovery reactor 5 for reacting magnesium with raw water to produce MAP; an additive injection line L50 for injecting an additive into an additive injection part; a raw water introduction line L4 for introducing the raw water into a raw water introduction part which is positioned above the additive injection part, to generate an upward flow therein; at least one of a pH adjusting source feeder, which is arranged on the additive injection line and used for feeding a pH adjusting agent as the additive, and a magnesium source feeder for feeding a magnesium source; agitation units 81, 91 which are arranged on the additive injection line and used for agitating/mixing the additive with the treatment water; a treated water line L5 through which the treated water to be withdrawn from the phosphorus recovery reactor is passed; and a first treated water reflux line L51 which is branched from the treated water line, connected to the additive injection line and used for refluxing a part of the treated water to the phosphorus recovery reactor together with the additive through the additive injection line to generate an upward flow therein. COPYRIGHT: (C)2011,JPO&INPIT